This invention relates generally to a method and apparatus for delivering a lower energy therapeutic pulse to a patient. Further this invention relates to a method and apparatus for shaping a delivered waveform by pulsing the energy delivered. More specifically, this invention relates to an electrotherapy method; and an apparatus for delivering electrotherapy to a patient. In particular, this invention relates to a method and apparatus for delivering a lower energy, therapeutically effective electrical waveform to a patient through a defibrillator. The method employed to lower the energy of the waveform can be applied to internal cardiac defibrillators (ICDs), automatic or semi-automatic external defibrillators (AEDs) and manual external defibrillators.
Sudden cardiac death is the leading cause of death in the United States. On average, 1000 people per day die; this translates into one death every two minutes. Most sudden cardiac death is caused by ventricular fibrillation (xe2x80x9cVFxe2x80x9d), in which the heart""s muscle fibers contract without coordination, thereby interrupting normal blood flow to the body. The only effective treatment for VF is electrical defibrillation, which applies an electrical shock to the patient""s heart. The electrical shock clears the heart of the abnormal electrical activity (in a process called xe2x80x9cdefibrillationxe2x80x9d) by depolarizing a critical mass of myocardial cells to allow spontaneous organized myocardial depolarization to resume.
To be effective, the defibrillation shock must be delivered to the patient within minutes of the onset of VF. Studies have shown that defibrillation shocks delivered within one minute after the onset of VF achieves up to a 100% survival rate. However, the survival rate falls to approximately 30% after only 6 minutes. Beyond 12 minutes, the survival rate approaches zero. Importantly, the more time that passes, the longer the brain is deprived of oxygen and the more likely that brain damage will result.
As would be expected, because external defibrillators deliver their electrotherapeutic pulses to the patient""s heart indirectly (i.e., from the surface of the patient""s skin rather than directly to the heart), they operate at higher energies, voltages and/or currents than ICDs. One consequence of using high energy, voltage and current is that external defibrillators have tended to be large, heavy and expensive, particularly due to the large size of the capacitors or other energy storage media required. Historically, the size and weight of external defibrillators has limited their utility for rapid response by emergency medical response teams. Additionally the higher energies used are associated with increased damage to the cardiac tissue.
Another disadvantage is that the shapes of the waveforms are a function of the components used to deliver the energy.
Yet another disadvantage of external defibrillators is that it may be subjected to extreme load conditions which could potentially damage the circuitry. For example, improperly applied defibrillator electrodes may create a very low impedance current path during the shock delivery, which could result in excessively high current within the waveform circuit. Thus, an external defibrillator has an additional design requirement to limit the peak current to safe levels in the waveform circuit, which is not normally a concern for implanted defibrillators.
Gliner et al., U.S. Pat. No. 5,607,454 entitled xe2x80x9cElectrotherapy Method and Apparatus,xe2x80x9d describes an external defibrillator which is capable of delivering an impedance compensated biphasic waveform. The use of a biphasic waveform considerably lowers the energy required to defibrillate a patient from the standard 200-300-360J used in monophasic external defibrillators to 150J. This enables the device to achieve a lower weight (4 lbs.) than possible for traditional monophasic devices, which typically weigh in excess of 8 lbs. The advancements taught by Gliner et al. are embodied in the Heartstream ForeRunner(copyright) AED.
In further support of using lower energies, evidence has shown that higher energies, such as those typically used in delivering a monophasic waveform, can result in damage to the heart tissuexe2x80x94particularly where successive shocks are needed to defibrillate a patient. The use of the lower energy, such as those employed when delivering an impedance compensated biphasic waveform (as described by Gliner et al.), reduces the likelihood of damage to the heart tissue. Evidence received from professionals in emergency medicine also indicates that the physiological effect on a patient receiving the same number of defibrillation shocks from a monophasic vs. a biphasic defibrillator is significant. Specifically, the use of low energy biphasic waveforms provides significantly less post-resuscitation myocardial dysfunction.
Sweeney et al. conducted a study of the effect of high-frequency monophasic rectangular pulses delivered directly onto the pericardium. Sweeney et al. xe2x80x9cDefibrillation Using A High-Frequency Series of Monophasic Rectangular Pulses: Observations and Model Predictionsxe2x80x9d J. Cardiovasc. Electrophys. 7(2): 1996. Sweeney concluded that defibrillation was possible using HF pulsed current waveform at frequencies as high as 20 kHz. From the results, Sweeney predicted that a 10 m-sec ascending ramp waveform will require 46% less energy than a 10 m-sec truncated exponential waveform.
Notwithstanding the great strides made in developing a lower energy, impedance compensated waveform. Improvements are still possible that would either lower the energy requirements even further without compromising the efficacy of the treatment or enable a variety of custom shaped waveforms to be delivered. What is needed, therefore, is a method of delivering energy to a patient that provides an efficacious therapeutic energy pulse while reducing the total amount of energy delivered. Further what is needed is a way to reduce the size of the components associated with the defibrillator.
In one embodiment, this invention provides a defibrillator and defibrillation method that delivers a lower energy waveform for administering a therapeutic shock to the cardiac muscle of a patient in VF. In another embodiment, the invention also allows the waveform to be shaped more accurately. In one specific embodiment, the invention provides a lower energy waveform that automatically compensates for patient-to-patient impedance differences in the delivery of electrotherapeutic pulses for defibrillation and cardioversion. In an even more preferred embodiment, the defibrillator has an energy source that may be discharged through electrodes on the patient to provide a biphasic voltage or current pulse. In one aspect of the invention, the first and second phase duration and initial first phase amplitude are predetermined values. This is achieved by duty cycling the voltage delivery so that it cycles between ON and OFF. Voltage cycling can be accomplished at a fixed frequency with a fixed pulse width. Alternatively, duty cycling of the voltage or current can be accomplished by providing a fixed frequency with a variable pulse width, or by providing a variable frequency with a fixed pulse width.
A method for delivering electrotherapy to a patient through electrodes, the method comprising the following steps: discharging an energy source across the electrodes to deliver electrical energy to the patient in at least one phase; wherein the discharging step is duty cycled during at least one phase at a frequency sufficient to generate a therapeutically efficacious voltage envelope. Additionally the discharging step may have a plurality of phases. The plurality of phases being biphasic. Optionally the discharging step may be cycled during more than one phase. Duty cycling can be performed at a constant frequency or at a variable frequency. Alternatively, or in addition, the pulse width of the duty cycle may be a constant width or a variable width. This method may be employed in an implantable cardiac defibrillator or an external defibrillator, more specifically an automatic external defibrillator.
Another method involves delivering electrotherapy to a patient through electrodes, the method comprising the following steps: discharging an energy source across the electrodes to deliver a electrical energy to the patient in at least one phase; wherein the discharging step is duty cycled during at least one phase at a frequency sufficient to generate a therapeutically efficacious average voltage. Additionally the discharging step may have a plurality of phases. The plurality of phases being biphasic. Optionally the discharging step may be cycled during more than one phase. Duty cycling can be performed at a constant frequency or at a variable frequency. Alternatively, or in addition, the pulse width of the duty cycle may be a constant width or a variable width. This method may be employed in an implantable cardiac defibrillator or an external defibrillator, more specifically an automatic external defibrillator.
An apparatus that performs these methods is also provided for. The apparatus delivers electrotherapy to a patient through one or more electrodes and comprises: a storage circuit operable to store electrical energy; a steering circuit coupled with the storage circuit, the steering circuit being adapted for coupling with the patient and operable to deliver the electrical energy from the storage circuit to the patient; and a switch operable to duty cycle the voltage delivered to the patient. As described in the method, the voltage duty cycle has a pulse width for each cycle and further wherein the voltage duty cycle is delivered at a frequency. Further, the steering circuit is capable of delivering energy to the patient in a plurality of phases. The switch is operable to duty cycle the voltage delivered to the patient at a constant frequency or a variable frequency.
The invention is described in more detail below with reference to the drawings.